This invention relates to a Rankine cycle heat exchanger (evaporator) using a low boiling point medium as a working fluid, in, for example a refrigerating apparatus and a nuclear power generating plant.
Typical prior art shell-tube type heat exchangers include a plurality of heat transfer tubes located in a cylindrical shell, with a medium, in a liquid state, being filled in the shell and with warm water being passed through the heat transfer tubes to allow a nucleate boiling heat transfer to take place on outer wall surfaces of the heat transfer tubes. In recent years, attention has been attracted to Rankine cycle generating plants which use a low boiling point medium as a working fluid so as to enable an effective use of low temperature energy, such as waste heat, geothermal heat, marine temperature differences, etc. To this end, it is desired that a heat exchanger of smaller size and higher performance than the shell-tube type heat exchanger be developed. Various types of heat exchanger have been proposed which might meet the needs.
To meet the above described needs in, for example, British Patent Specification 261,731 a heat exchanger is proposed which includes a plurality of vertically arranged heat transfer tubes, with a medium, in a liquid state, flowing downwardly in the form of a liquid film along an outer wall surface of each of the tubes to accelerate vaporization of the liquid. In this type of heat exchanger, it is necessary to supply a sufficiently large quantity of liquid from above to prevent the wall surfaces of lower portions of the heat transfer tubes from becoming dry. As a result, a mean thickness of the liquid film on each heat transfer tube of a great length can not be reduced to a satisfactorily low level. Also, at working points other than those of design load, a liquid flowing downwardly through a supply port located at a higher elevation might reach a supply port located at a lower elevation before vaporization of all the liquid takes place and the liquid flowing downwardly and the newly supplied liquid through the supply port would flow in a two-phase stream flow, thereby greatly reducing the performance of the evaporator. The liquid flowing downwardly from the supply port located at the higher elevation might be completely evaporated before reaching the supply port at the lower elevation, thereby also reducing the performance of the evaporator due to the presence of a dry heat transfer surface. Thus, it would be impossible for this type of heat exchanger to achieve a marked improvement in the performance of the evaporator. Even if high performance could be obtained with respect to a design load, it would be impossible to maintain the performance of the evaporator at the planned level by suitably coping with changes that the load applied to the evaporator might undergo.
This invention has as its object the provision of a heat exchanger compact in size, low in resistance offered to the flow of a two-phase mixture of liquid and gas of a medium that vaporizes and high in heat exchanging performance, and which is capable of maintaining its heat exchanging performance at a high level even if the capacity of a heat source undergoes a change and a load other than a design load is applied to the heat exchanger.
In accordance with the present invention, a liquid film evaporation type heat exchanger is provided wherein a medium, in a liquid state, flows downwardly in the form of a film along heat transfer surfaces, with the heat exchanger comprising at least two heat transfer units arranged at locations spaced apart from each other in a direction in which the medium in the liquid state flows. Each of the heat transfer units includes a plurality of planar heat transfer ducts each formed with a plurality of warm water passageways extending perpendicular to the direction of flow of the medium in the liquid state, and a plurality of liquid distributing beams each formed with a plurality of cutouts. The planar heat trasfer ducts and the liquid distributing beams are alternately arranged in the respective units, and a port, for releasing the medium in a gaseous state, is positioned between the adjacent heat transfer units.